1. Field of the Disclosure
Exemplary embodiments of the present disclosure relate to a fixing device and an image forming apparatus including the fixing device, and more specifically, to a fixing device that applies heat and pressure to a recording medium at a nip formed between a fixing member and a pressing member to fix an image on the recording medium, and an image forming apparatus including the fixing device.
2. Description of the Background
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction apparatuses having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image carrier; an optical writer emits a light beam onto the charged surface of the image carrier to form an electrostatic latent image on the image carrier according to the image data; a development device supplies toner to the electrostatic latent image formed on the image carrier to make the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image carrier onto a recording medium or is indirectly transferred from the image carrier onto a recording medium via an intermediate transfer member; a cleaner then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.
Such fixing device may include a cylindrical metal member to heat the fixing device effectively to shorten a warm-up time or a time to first print (hereinafter also “first print time”). Specifically, the metal member provided inside a loop into which an endless fixing belt is formed, facing the inner circumferential surface of the fixing belt. The metal member is heated by a built-in or external heater so as to heat the fixing belt. A pressing roller presses against the outer circumferential surface of the fixing belt at a position corresponding to the location of the metal member inside the loop formed by the fixing belt to form a nip between the fixing belt and the pressing roller through which the recording medium bearing the toner image passes. As the recording medium bearing the toner image passes through the nip, the fixing belt and the pressing roller apply heat and pressure to the recording medium to fix the toner image on the recording medium.
To maintain a desired surface temperature (fixing temperature) of the fixing belt, a contact-type temperature sensor (temperature detector) is provided at the outer circumferential surface of the fixing belt. The heater is turned on and off in accordance with detection results of the temperature sensor.
For example, JP-2008-146010-A proposes a fixing device including a temperature sensor (temperature detector) to detect a temperature of the fixing belt. In this case, the temperature sensor is pressed against an inner circumferential surface of a metal member and a resistant heat generator serving as a heater.
Further, JP-2009-003410-A proposes a fixing device including a stationary member (contact member) against which the pressing roller is pressed to form a nip portion and a reinforcement member to reinforce the stationary member.
In the above-described fixing devices, because the contact-type temperature sensor (thermistor) is disposed in contact with the outer circumferential surface of the fixing belt, the outer circumferential surface of the fixing belt gets worn and deteriorates, which might cause a degraded fixing image. In particular, if the fixing belt is not reliably maintained in a substantially circular loop, the contact-type temperature sensor needs to be pressed against the fixing belt with a relatively large pressing force to ensure reliable contact with the fixing belt, which may make the above-described challenge non-negligible.
To deal with such a challenge, it is conceivable to use a non-contact-type temperature sensor, such as a thermopile. However, such a non-contact-type temperature sensor may be quite expensive. Moreover, if the contact-type temperature sensor is disposed at a non-sheet-pass area of the fixing belt, that is, an area of the fixing belt over which a recording sheet is not usually conveyed in the course of normal image formation, then the size of the fixing device is enlarged laterally. A similar outcome occurs if the temperature sensor is disposed at the outer circumferential surface side of the fixing belt, in which case the fixing device is enlarged radially.
To deal with such challenges, it is conceivable to locate the temperature sensor inside the pipe-shaped metal member. In such a case, however, it is unclear whether the surface temperature of the fixing belt can be detected with high sensitivity and precision. Further, with such a configuration, the temperature sensor might be directly heated by the heater disposed inside the pipe-shaped metal member, damaging the temperature sensor.